The present invention relates to improvement in the performance of a reverse osmosis composite membrane such as performance stability and fouling tolerance for selectively separating the components of a liquid mixture. More particularly, the present invention relates to a reverse osmosis composite membrane having a high salt rejection, a high chlorine tolerance, and a high fouling tolerance, which comprises a polymer thin film on the reverse osmosis composite membrane, and to a reverse osmosis treatment method for water using the same.
Such a reverse osmosis composite membrane is suitable for manufacturing ultrapure water, desalinating brackish water, and the like, and it also can contribute to the removal and recovery of the contaminating sources or effective substances from a soil or the like, the cause of pollution in a dyeing waste water system or an electrochemical deposition paint waste water system to implement a waste water recycling system. In particular, it can operate stably for a long period with respect to the quality of water containing various membrane-fouling substances, such as surfactants and transition metal components including iron, which cause a decrease in flux.
Examples of the industrially utilized reverse osmosis membranes include Loeb type membranes described in U.S. Pat. Nos. 3,133,132 and 3,133,137 as asymmetric membranes made of cellulose acetate. On the other hand, reverse osmosis composite membranes, in which an active thin film substantially having a selective separation property is formed on a microporous support membrane, are known as reverse osmosis membranes having a different structure from the asymmetric reverse osmosis membranes.
Presently, a number of such reverse osmosis composite membranes, in which a thin film of polyamide obtained by interfacial polymerization of polyfunctional aromatic amine and polyfunctional aromatic acid halide is formed on a support membrane, are known (for example, Publication of Japanese Patent Application (Tokkai Sho) No. 55-147106, Publication of Japanese Patent Application (Tokkai Sho) No. 62-121603, Publication of Japanese Patent Application (Tokkai Sho) No. 63-218208, and Publication of Japanese Patent Application (Tokkai Hei) No. 2-187135). Also, those having a thin film of polyamide obtained by interfacial polymerization of polyfunctional aromatic amine and polyfunctional alicyclic acid halide formed on a support membrane are known (for example, Publication of Japanese Patent Application (Tokkai Sho) No. 61-42308).
In addition, various methods for after treatment of the reverse osmosis membrane are disclosed. For example, methods using various polymers as a protective layer are disclosed (for example, Publication of Japanese Patent Application (Tokkai Sho) No. 51-13388, Publication of Japanese Patent Application (Tokkai Sho) No. 53-16372, Publication of Japanese Patent Application (Tokkai Sho) No. 62-197105, and Publication of Japanese Patent Application (Tokko Hei) No. 7-90152) Recently, it has been expected to apply a reverse osmosis membrane to a treatment for water containing fouling substances such as various surfactants, for example, sewage. In addition to the high performance of the reverse osmosis membrane (a high salt rejection and a high water permeability), a high fouling resistance is required to maintain the desired flux for a long period. The above reverse osmosis membranes and the conventional after treatment methods are not sufficient to satisfy these two requirements. Therefore, a reverse osmosis composite membrane having a higher performance has been sought.
One fouling mechanism includes the charge condition of the membrane. For example, the surface of a cross-linked polyamide reverse osmosis membrane obtained by interfacial polymerization of polyfunctional aromatic amine and polyfunctional alicyclic acid halide has a negative charge due to the residual carboxylic acid. The membrane surface having negative charge adsorbs, for example, cationic fouling substances, decreasing the flux. Therefore, a membrane has been required that is neutral in charge and has a high water permeability and a high salt rejection.
It is an object of the present invention to provide a reverse osmosis composite membrane that has a high salt rejection, a high water permeability, and a high fouling tolerance and permits practical desalination at a low pressure, and a reverse osmosis treatment method for water using the same.
In order to achieve the above object, the present invention provides a reverse osmosis composite membrane comprising a sponge layer, and a separation layer (also referred to as a skin layer) formed on a surface of the sponge layer, wherein at least one substance selected from the group consisting of an electrically neutral organic substance and an electrically neutral polymer is present in the separation layer or a surface of the separation layer is coated with at least one substance selected from the group consisting of an electrically neutral organic substance and an electrically neutral polymer, and wherein a specific surface area of the layer in which the at least one substance is present or the separation layer before the surface coating is in the range of 2 to 1,000. The specific surface area is defined by the following expression: the specific surface area of the separation layer=(the surface area of the separation layer)/(the surface area of the support).
The surface area of the separation layer represents the area of the surface to be in contact with a feed liquid. The surface area of the support indicates the surface area of a support membrane such as polysulfone, with which the sponge layer under the separation layer is in contact. The specific surface area of the separation layer is 2 or more when the surface of the support membrane is relatively flat and the surface of the separation layer is rough and has wrinkles. The surface area and the specific surface area are measured by a general measurement device, for example, a surface area measurement device, a specific surface area measurement device, a scanning electron microscope, a transmission electron microscope, or an atomic force microscope.
In the reverse osmosis composite membrane, it is preferable that the specific surface area of the layer in which the at least one substance is present or the separation layer before the surface coating is in the range of 3 to 500. When the specific surface area is 3 or more, the water permeability is improved. If the specific surface area is more than 500, the strength of the separation layer decreases.
In the reverse osmosis composite membrane, the specific surface area of the separation layer after the surface coating is preferably reduced to 90% or less of the specific surface area of the separation layer before the surface coating, and more preferably, 60% or less. If the specific surface area of the separation layer after the surface coating is more than 90% of that of the separation layer before the surface coating, the surface is not sufficiently covered, preventing a stable fouling tolerance for a long period.
In the reverse osmosis composite membrane, it is preferable that the at least one substance selected from the group consisting of an electrically neutral organic substance and an electrically neutral polymer is an organic substance or a polymer that has a nonionic hydrophilic group. Electrical neutrality is preferred for controlling the electrical adsorption by the membrane of membrane-fouling substances having a charge group present in water. If the organic substance or the polymer has a nonionic hydrophilic group (for example, a xe2x80x94OH group) in addition to the electrical neutrality, adsorption due to a hydrophobic interaction on the membrane is controlled when the membrane-fouling substances have a hydrophobic group.
In the reverse osmosis composite membrane, it is preferable that the organic substance or the polymer that has a nonionic hydrophilic group is polyvinyl alcohol that is water-insoluble at 25xc2x0 C. and is water-soluble at 80xc2x0 C. Such polyvinyl alcohol controls the adsorption of the membrane-fouling substances. On the other hand, polyvinyl alcohol that is water-insoluble at more than 80xc2x0 C. has a small number of alcohol groups, so that such polyvinyl alcohol does not tend to control the adsorption of the membrane-fouling substances.
In the reverse osmosis composite membrane, it is preferable that the polyvinyl alcohol has a saponification degree of 95% or more, because a sufficient number of alcohol groups can be obtained.
In the reverse osmosis composite membrane, it is preferable that the at least one substance selected from the group consisting of an electrically neutral organic substance and an electrically neutral polymer coating the surface of the separation layer is an organic substance or a polymer that has a nonionic hydrophilic group, and has a thickness in the range of 0.001 to 1 xcexcm. If the thickness is more than 1 xcexcm, the water permeability obtained after the coating decreases significantly. If the thickness is less than 0.001 xcexcm, a uniform coating will be difficult.
In the reverse osmosis composite membrane, it is preferable that the reverse osmosis composite membrane has a flux of 0.6 [m3/m2/day] or more in a reverse osmosis test conducted by feeding a 500 ppm NaCl solution at a pressure of 7.5 kgf/cm2 and a temperature of 25xc2x0 C. to ensure a sufficient flux after the coating.
In the reverse osmosis composite membrane, it is preferable that the reverse osmosis composite membrane has a flux of 0.6 [m3/m2/day] or more in a reverse osmosis test conducted by feeding a 1500 ppm NaCl solution at a pressure of 15 kgf/cm2 and a temperature of 25xc2x0 C. to ensure a sufficient flux at a practical level.
In the reverse osmosis composite membrane, it is preferable that the reverse osmosis composite membrane is formed of aromatic polyamide. Here, the aromatic polyamide refers to polyamide in which at least one component selected from the group consisting of an acid component and an amine component is aromatic. The preferred aromatic polyamide is a completely aromatic polyamide comprising an aromatic acid component and an aromatic amine component. Such aromatic polyamide can maintain a high water permeability and a high salt rejection.
The present invention provides a reverse osmosis treatment method for water comprising contacting raw water with a reverse osmosis composite membrane comprising a sponge layer and a separation layer formed on a surface of the sponge layer, wherein at least one substance selected from the group consisting of an electrically neutral organic substance and an electrically neutral polymer is present in the separation layer or a surface of the separation layer is coated with at least one substance selected from the group consisting of an electrically neutral organic substance and an electrically neutral polymer, and wherein a specific surface area of the layer in which the at least one substance is present or the separation layer before the surface coating is in the range of 2 to 1,000.
In the method, the reverse osmosis composite membrane is useful even for treating water containing a surfactant. In the conventional reverse osmosis composite membranes, the membrane adsorbs a surfactant, so that a stable performance cannot be obtained. In the reverse osmosis composite membrane of the present invention, the adsorption of the surfactant by the membrane is controlled. Therefore, a stable reverse osmosis treatment can be conducted for water containing a surfactant without decreasing the separation performance.
In the method, it is preferable that the content of the surfactant is in the range of 0.01 ppm to 20 wt. %. In the method of the present invention, the content of the surfactant can be in any range. If the content is in the range of 0.01 ppm to 20 wt. %, a sufficient surface potential property of the present invention can be provided.
In the method, the reverse osmosis composite membrane is useful even for treating water containing a transition metal component. In the conventional reverse osmosis composite membranes, the membrane adsorbs a transition metal component, so that a stable performance cannot be obtained. In the reverse osmosis composite membrane of the present invention, the adsorption of the transition metal component by the membrane is controlled. Therefore, a stable reverse osmosis treatment can be conducted for water containing a transition metal component without decreasing the separation performance.
In the method, it is preferable that the transition metal component is iron, because the reverse osmosis composite membrane of the present invention can control the adsorption of the iron component sufficiently.
In the method, it is preferable that the content of the transition metal component is in the range of 0.01 ppm to 20 wt. %. In the method of the present invention, the content of the transition metal component can be in any range. If the content of the transition metal component is in the range of 0.01 ppm to 20 wt. %, a sufficient surface potential property of the present invention can be provided.
In the method, the flux does not decrease even when treating raw water containing a fouling substance (including a surfactant) having 20 to 30 ppm of TOC (total organic carbon). Therefore, a practical water treatment can be performed. On the other hand, the flux decreases when treating such raw water in the conventional technology. Therefore, a practical water treatment cannot be performed.
According to the reverse osmosis composite membrane and the reverse osmosis treatment method using the same of the present invention as described above, a high salt rejection, a high water permeability, and a high fouling tolerance is provided, and a practical desalination can be conducted at a low pressure.